(1) Field of the Invention
The present invention relates to an electromagnetic induction heating device that controls electric power supplied to a resonant circuit that is connected to a switching element by performing control of ON and OFF switching by a switching element, and has an inductor and a capacitor, and that performs electromagnetic induction of heating a component subject to heating through electromagnetic flux in the inductor. In particular, the present invention pertains to technology for preventing the occurrence of noise produced when the electromagnetic induction heating device performs intermittent control of switching the switching element ON and OFF when low electric power is supplied, which is likely to cause switching loss.
(2) Description of the Related Art
Electromagnetic induction heating has come to be used for a printer, copier, or other image formation apparatus, as a heating method that provides the image formation apparatus with a shorter warm-up period and energy savings.
FIG. 10 illustrates a specific example of an electromagnetic induction heating device for an image formation apparatus using a conventional electromagnetic induction heating method. As shown, an electromagnetic induction heating device 100 is connected to a commercial power source (e.g., AC 100V) 110 serving as a source of electric power, a rectifier circuit 120, an electric power detection circuit 130, an inverter circuit 140, and an electromagnetic induction heating control unit 150. The commercial power source 110 supplies alternating current that is converted into direct current by the rectifier circuit 120 and subsequently supplied to the inverter circuit 140.
The electric power detection circuit 130 detects electric power in the rectifier circuit 120 and outputs detection results to the electromagnetic induction heating control unit 150. The inverter circuit 140 includes a resonant circuit 141 having an inductor 1411 and a capacitor 1412 connected in parallel, and a switching element 142 connected in series to the resonant circuit 141. The inverter circuit 140 repeatedly operates the supply of direct current to the resonant circuit 141 to be ON and OFF as the switching element 142 is controlled to be ON and OFF, supplies high-frequency electric power to the inductor 1411, and causes electromagnetic induction heating in the non-diagrammed component subject to heating (e.g., a fixing roller) that is electromagnetically connected to the inductor 1411.
The electromagnetic induction heating control unit 150 performs pulse width modification (hereinafter, PWM) control of controlling the duty cycle of the switching element 142, thereby controlling the electric power supplied to the resonant circuit 141. The duty cycle is a proportion (percentage) of time during which the switching element is ON relative to the PWM signal cycle. A higher duty cycle produces control such that more electric power is supplied. Conversely, a lower duty cycle produces control such that less electric power is supplied.
FIG. 11 illustrates the relationships between voltage and current applied to the switching element when the switching element is switched ON and OFF during PWM control (see Japanese Patent Application Publication No. 2009-204717). Section (a) of FIG. 11 represents a switching signal indicating whether the switching element is ON or OFF. Sections (b) and (c) of FIG. 11 respectively indicate the changes in voltage and current applied to the switching element.
As shown in sections (a) through (c) of FIG. 11, the PWM control beneficially executes zero-crossing control of switching the switching element ON and OFF such that the applied voltage and current are approximately zero. This approach enables prevention of the loss of electric power occurring in the switching element (hereinafter termed switching loss) and of the overheating or breakage therein.
However, when low electric power is supplied to the resonant circuit (e.g., when the temperature of the fixing roller is sufficiently high such that there is no great difference from the target temperature), the switching element is ON for a shorter time during the PWM control. This causes less electric power to be accumulated in the inductor of the resonant circuit during the ON time. As a result, and as shown in sections (d), (e), and (f), the vibration amplitude of the voltage applied to the switching element is reduced and the timing becomes such that the switching element is switched ON before the voltage decreases all the way to zero, such that zero-crossing control cannot be performed.
Thus, when the switching element is switched ON and OFF with timing on the order of microseconds, switching loss occurs every time the switching element is switched ON, which produces increasing switching loss and is likely to cause breakage of the switching element through the production of heat that accompanies switching loss.
Accordingly, intermittent PWM control is executed so as to intermittently execute PWM control at regular time intervals. Thus, when the PWM control is being executed, the time during which the switching element is ON is made longer than is the case in the above-described continuous PWM control. This enables the prevention of switching loss by increasing the electric power supplied to the resonant circuit. Also, this approach provides a stop period during which the PWM control is stopped, and enables electric power supplied in excess to be cancelled out by lengthening the ON time.
However, when the above-described intermittent PWM control is performed, dramatic variations in electromagnetic flux are produced during the transition from a stop period during which the PWM control is stopped to an execution period during which the PWM control is executed, and the transition from the execution period to the stop period. Thus, the component subject to heating (i.e., the fixing roller) is repeatedly deformed, which results in a problem of noise production.
In order to prevent the problem of noise production, the supply of electric power to the resonant circuit may be modified to be gradual during the start and stop of the execution period for the PWM control, thus preventing the dramatic change in electromagnetic flux (see also Japanese Patent Application Publication No. 2011-253682). However, although this approach does prevent the occurrence of noise, a further problem occurs in that the switching loss is increased.